ARCA. 



71 



is much thicker, and more testaceous. The species are found in 

 the Mediterranean, European, and West Indian Seas. A. Peter- 

 soni, fig. 254. 



APLYSIACEA. Bl. The second family of the order Monopleuro- 

 branchiata, Bl. The animals composing this family are either 

 destitute of shells, or are provided with internal ones, which are 

 flat, open, oblique, with the apex or nucleus slightly incurved, 

 not distinctly spiral. This family contains the genera, Aplysia 

 and Dolabella. The first sub-spiral, with the apex terminal ; 

 shell thin, horny. Fig. 254. The second the same, but thick 

 and shelly. Fig. 255. 



APOLLON. Montf. Ranella Ranina, Auct. Placed by De 

 Blainville in that division of Ranella, which is characterized as 

 being umbilicated. Fig. 393. 



APOROBRANCHIATA. Bl. The first order of the second section 

 of Paracephalophora Monoica, Bl. The Thecosmata is the only 

 family of this order containing any approach to shells, these are 

 Hyalaea and Cymbulia. 



APORRHAIS. Petiver. A genus formed of Rostellaria Pes- 

 pelicani, Auct. (fig. 404) and similar species. Although the shell 

 presents no characters to distinguish it generically from Rostel- 

 laria, those who have examined the soft parts are convinced that 

 it is distinct. Of the three species now known and figured in 

 part I. of Thesaurus Conchyliorum, by the Author, one is com- 

 mon on our own coast, and in the Mediterranean ; also North 

 America. See Rostellaria. 



AQUATIC. A term applied by some authors to those species of 

 Molluscous animals, which inhabit fresh water, either in rivers, or 

 salt water standing pools, as distinguished from the marine or 

 Mollusca. See Fresh-water. 



AQUILLUS. Montf. Triton Lampas, Cutaceus, &c. Auct. 

 Placed by De Blainville in the division of the genus Triton, 

 which is described as having a short spire, being covered with 

 tubercles and umbilicated. Triton Cutaceus, fig. 399. 



ARCA. At *t. (Anglice, a boat.) Fam. Arcacea, Lam. — Bescr. 

 Obliquelj transverse, subquadrate, equi valve, or nearly so, 



